Lentiviruses of sheep, goats and horses have been shown to represent a genetically distinct group of retroviruses to which the AIDS retroviruses (HTLV-III/LAV) are highly related. Molecular clones of caprine arthritis encephalitis virus (CAEV) and equine infectious anemia virus (EIAV) were shown to be highly homologous in their polymerase genes and less so in their gag genes. Nucleotide sequence analysis of a complete molecular clone of EIAV revealed that the genetic organization of the viral genome is very similar to that of visna and HTLV-III. Current efforts to express EIAV gene products in prokaryotic and eukaryotic vectors and to analyze genetic and antigenic drift in the viral glycoprotein should lead to better diagnostic tests and an understanding of the requirements for a vaccine. At the same time, highly potent inhibitors (2',3'-dideoxynucleosides) of the in vitro replication of lentiviruses have been identified and in vivo testing involving pharmacokinetics, long-term toxicity, and effectiveness in clearing animals of retrovirus are being initiated.